The present invention relates to a rope tensioner which may be used to tension a rope operated switch assembly. Such an assembly may be used, particularly but not exclusively for controlling the power supply to kinetic machinery.
Rope operated switch assemblies are generally fitted in proximity to a machine or around any area which requires protection, and comprise two safety switches and a rope extending between the switches such that the electrical power supply may be turned off when the rope is pulled or slackened (e.g. if the rope is cut). In some applications, only one switch is provided, the end of the rope being connected to the single switch and the other end being connected to for example a spring secured to a fixed point. The rope is generally a plastics-coated metal wire or cable, and extends around the machine so that an operator can easily reach it from any position adjacent the machine. The term xe2x80x9cropexe2x80x9d used hereinafter is intended to mean any elongate flexible element that is suitable for using in a rope operated switch assembly, for example, metal cable, cord formed of twisted elements or fibres such as wire, polymeric material, etc., which may optionally be coated with a plastics material.
The rope must be installed at the correct tension so that the machinery can be operated but so that a relatively small change in the tension of the rope will actuate the switch assembly. This is important because an injured operator may not be able to pull the rope with much force.
The installation of the rope at the correct tension is difficult to achieve. The rope must first be installed between the two switches, and is then pulled into an approximation of the correct tension. A turnbuckle is installed in the centre of the rope by cutting the rope and fixing the turnbuckle between the rope sections using thimbles and cable clamps. Once installed, the tension in the rope is set by adjusting eye bolts on the turnbuckle until the rope switches are pulled into a xe2x80x9crunxe2x80x9d position, that is, with safety contacts in the switches made. The turnbuckle then allows small alterations to be made to the tension of the rope to allow for expansion or contraction of the rope due to, for example, temperature differences.
Placement of the turnbuckle in the rope is a time-consuming and often difficult process, and can take up a large proportion of the total time taken to install the rope switch assembly. The setting of the correct tension in the rope is particularly difficult, as the switches must be continually checked to ensure that the switch mechanisms are in the correct position in order that the rope is not set at an incorrect tension. As the turnbuckle ideally is placed towards the centre of the rope to allow for even tensioning, the turnbuckle may be some distance from the switches.
In addition, the turnbuckle only allows small differences in tension in the rope to be corrected for. If the tension alters by more than can be dealt with using the turnbuckle, the slack or expansion must be dealt with by movement of the thimbles and cable clamps along the rope segments. A large alteration in tension can occur, for example, due to large temperature fluctuations in different seasons, especially on long runs of rope.
It is an object of the present invention to obviate or mitigate such disadvantages with prior art systems.
According to a first aspect of the present invention there is provided a rope operated switch assembly comprising a rope extending to at least one switch, the switch being actuable to switch between first and second conditions on a change in tension of the rope, and a tensioner for setting the tension of the rope, wherein the tensioner comprises a body, a rotatable member which engages the rope, means for rotating the member relative to the body to adjust tension in the rope by winding the rope around the member, and locking means to lock the member in position relative to the body.
The tensioner may be simply threaded or otherwise installed onto the rope during installation of the rope operated switch assembly, without the need to cut the rope. The installation is thus much simpler than with prior art switch assemblies.
The locking means preferably comprises a ratchet and pawl. Preferably, means displaceable relative to the body are provided to move the pawl out of engagement with the ratchet, thereby unlocking the shaft to release the tension in the rope. The displaceable means may comprise a screw that may be screwed into the body.
A plate is preferably provided to separate the tensioner into two compartments, the rope extending into one compartment, and the locking means being provided in the other compartment. This means that the rope does not become entangled in the locking means whilst the tensioner is being installed along the rope.
The tensioner is preferably provided adjacent the switch, meaning that the installation may be effected quickly as the operator can easily check the state of the adjacent switch as the tension in the rope is altered.
According to a second aspect of the present invention there is provided a method of installing a rope operated switch assembly comprising connecting the rope to at least one switch which is actuable to switch between first and second conditions on a change in tension of the rope, the rope being initially installed such that an approximation of the correct tension is achieved in the rope, and increasing the tension in the rope by means of a tensioner, wherein the tensioner comprises a body and a rotatable member which engages the rope, the tension in the rope being increased by rotating the rotatable member relative to the body such that the rope is wound around the member, and locking the rotatable member in position relative to the body after the rope has been tensioned.
The tension of the rope is preferably monitored as the tension is increased by viewing a tension indicator provided on the at least one switch. The tensioner is preferably placed close to an end of the rope, adjacent to the at least one switch, for ease of viewing of the tension indicator provided on the switch.
Unlocking means are preferably provided in the tensioner so that the tension of the rope may be released.
According to a third aspect of the present invention, there is provided a rope tensioner comprising a body defining a passage through which a rope can be inserted to project from each end of the passage, a member rotatable relative to the body and formed to engage a rope extending through the passage such that rotation of the member causes the rope to be wound around the member, and means for locking the member in position relative to the body to resist unwinding of the rope from the member.
The means for rotating the rotatable member around which the rope is wound may be a drive member mechanically coupled by a gear to the rotatable member. The drive member may be a worm gear meshed with a gear supported by the rotatable member. The locking means may be provided by providing a gear system with a mechanical advantage such that unwinding of the rope from the rotatable member is prevented unless the drive member is rotated to drive the rotatable member in a direction to unwind the rope.
The rotatable member is preferably a shaft defining an aperture aligned with openings in the tensioner body.
The locking means preferably comprises a ratchet and pawl assembly.
Preferably the tensioner comprises means for releasing the locking means.